This application claims the priority benefit of Taiwan application serial no. 88123298, filed Dec. 30, 1999.
1. Field of the Invention
The present invention relates to a method of forming a pattern. More particularly, the present invention relates to a method of forming a rectangular pattern.
2. Description of the Related Art
Photolithographic and etching operations are the two most commonly used processes for forming patterns on a semiconductor wafer. In a conventional photolithographic process, photoresist material is first deposited over a substrate. The photoresist material is then exposed to light through a photomask so that the pattern on the photomask is transferred to the photoresist layer. Finally, the exposed photoresist layer is developed to form a patterned photoresist layer.
Due to fierce competition in the market place, semiconductor manufacturers are all aiming for higher level of integration. Various mechanical means for increasing the level of integration are thereby developed. As VLSI technologies continue to progress, all kinds of problems resulting from a reduction of line width must be solved. To resolve the problems arising in a photolithographic operation, various types of exposure stations using a variety of light sources including G-line, I-line, deep ultraviolet (DUV) and laser are now available. Nevertheless, there are still some limitations to forming perfect rectangular patterns.
FIG. 1 is a sketch showing the rounded comers of an originally rectangular pattern after a conventional photolithographic operation. As shown in FIG. 1, the four right-angled comers of a rectangular pattern 100 are truncated into a pseudo-rectangular pattern with four rounded corners 120 after a pattern transfer operation. In the fabrication of deep sub-micron devices, comer rounding of rectangular patterns is likely to intensify and production yield is likely to drop. The rounding of corners will improve if a better exposure station such as a deep ultraviolet stepper or a machine using an optical proximity correction (OPC) method is used. However, choosing an ultraviolet stepper or a machine that employs the OPC method can at most reduce the degree of rounding. To eliminate comer rounding altogether, some fundamental processing changes have to be introduced.
The invention provides a patterning method suitable for forming a rectangular pattern in a material layer above a substrate. A hard mask layer is formed over the material layer. A first patterned photoresist layer is formed over the hard mask layer. A first etching operation is next conducted to remove a portion of the hard mask layer not covered by the first patterned photoresist layer so that the pattern in the first photoresist layer is transferred to the hard mask layer. The first photoresist layer is removed. A second patterned photoresist layer is formed over the substrate. A portion of the pattern in the second photoresist layer overlaps with portions of the pattern in the first photoresist layer to form overlapping regions. These overlapping regions form a pattern identical to the desired pattern in the material layer. While using the second patterned photoresist layer and the exposed hard mask as an etching mask, a second etching operation is conducted. Ultimately, a portion of the material layer covered by neither the patterned second photoresist layer nor the hard mask layer is removed. Hence, the desired pattern is transferred to the material layer.
According to a first embodiment of this invention, the aforementioned method of patterning the material layer can be applied to form a rectangular pattern in the material layer. The method includes forming a patterned first photoresist layer having a linear opening over the material layer. One side of the linear opening and a first side of the desired rectangular opening in the material layer are parallel to each other, and the linear opening corresponds in position to the desired rectangular opening in the material layer. The linear opening has a length greater than the first side of the desired rectangular opening and a width identical to the width of the second side of the desired rectangular opening. After carrying out an etching operation to transfer the linear opening pattern in the first photoresist layer to the material layer, a patterned second photoresist layer is formed over the material layer. The second photoresist layer also contains a linear opening that corresponds in position to the desired rectangular opening in the material layer. However, one side of this linear opening is parallel to the second side of the desired rectangular opening. The linear opening has a width identical to the second side of the desired rectangular opening and a length greater than the second side of the desired rectangular opening. After a second etching operation is carried out to transfer the pattern in the second photoresist layer to the material layer, a rectangular opening pattern is transferred to the material layer.
The material layer having a rectangular opening therein can be used as a mask layer for forming a rectangular isolation structure. For example, while using the material layer as a mask, thermal oxidation of a substrate can be carried out to form a device isolation structure. Alternatively, the substrate can be etched to form a trench while using the material layer as an etching mask so that a rectangular trench isolation structure may subsequently form in the substrate.
The invention also provides a second patterning method suitable for forming a rectangular pattern in a material layer above a substrate. A hard mask layer is formed over the material layer. A first patterned photoresist layer is formed over the hard mask layer. A first etching operation is conducted to remove a portion of the hard mask layer not covered by the first photoresist layer so that the pattern on the first photoresist layer is transferred to the hard mask layer. The first photoresist layer is removed. A second patterned photoresist layer is formed over the substrate. A portion of the pattern on the second photoresist layer overlaps with portions of the pattern on the first photoresist layer to form overlapping regions. These overlapping regions form a pattern identical to the desired pattern in the material layer. While using the second patterned photoresist layer as an etching mask, a second etching operation is conducted so that a portion of the hard mask layer not covered by the patterned second photoresist layer is removed. Hence, the desired pattern is transferred to the hard mask layer. The second photoresist layer is removed, and then a third etching operation is conducted to transfer the desired pattern to the material layer while using the patterned hard mask layer as an etching mask.
According to a second embodiment of this invention, the aforementioned method of patterning the material layer can be applied to the formation of a rectangular island pattern in the material layer. The method includes forming a patterned first photoresist layer over the material layer. The pattern in the first photoresist layer is a linear island having one side parallel to a first side of the desired rectangular island pattern. The linear island is formed in a corresponding position above the desired rectangular island. Length of the linear island in the first photoresist layer is greater than the first side of the desired rectangular island pattern while width is identical to the second side of the desired rectangular island pattern. After carrying out a first etching operation to transfer the pattern in the first photoresist layer onto the material layer, a patterned second photoresist layer is formed over the material layer. The pattern in the second photoresist layer is in the shape of a rectangular island formed in a corresponding position above the desired rectangular island pattern. The first side of this rectangular island has a length identical to the first side of the desired rectangular island pattern while the second side has a length greater than the second side of the desired rectangular island pattern. A second etching operation is then carried out to transfer the pattern in the second photoresist layer onto the hard mask layer. Finally, a third etching operation is conducted to transfer the patterned hard mask onto the material layer, forming the desired rectangular island pattern in the material layer.
The rectangular island pattern on the material layer can be used for patterning a silicon nitride hard mask layer to form a rectangular-shaped active region in a substrate. For example, in local oxidation or shallow trench isolation to form an isolation structure that defines a rectangular active region, the rectangular island pattern in the material layer can be transferred to the silicon nitride layer. The silicon nitride layer is then used to perform a thermal oxidation. Alternatively, the silicon nitride layer can be used as a mask in an etching process.
Accordingly, the present invention provides a method capable of eliminating the rounding of comers after a rectangular pattern is transferred from a photomask to a photoresist layer in a photolithographic operation. Hence, production yield is increased.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.